up to five timeslarger than those generated in commerce and industry(Patel et al. 2000) and so in order to achieve highoverall recycling rates, post-consumer as well aspost-industrial waste need to be collected and recycled.In some instances recovered plastic that is notsuitable for recycling into the prior application is usedto make a new plastic product displacing all, or aproportion of virgin polymer resin—this can also beconsidered as primary recycling. Examples are plasticcrates and bins manufactured from HDPE recoveredfrom milk bottles, and PET fibre from recoveredPET packaging. Downgrading is a term sometimesused for recycling when recovered plastic is put intoan application that would not typically use virginpolymer—e.g. ‘plastic lumber’ as an alternative tohigher cost/shorter lifetime timber, this is secondaryrecycling (ASTM Standard D5033).Chemical or feedstock recycling has the advantageof recovering the petrochemical constituents of the poly-mer, which can then be used to re-manufacture plasticor to make other synthetic chemicals. However,while technically feasible it has generally been found tobe uneconomic without significant subsidies because ofthe low price of petrochemical feedstock comparedwith the plant and process costs incurred to producemonomers from waste plastic (Patel et al. 2000). Thisis not surprising as it is effectively reversing the energy-intensive polymerization previously carried out duringplastic manufacture.Feedstock recycling of polyolefins through thermal-cracking has been performed in the UK through afacility initially built by BP and in Germany byBASF. However, the latter plant was closed in 1999(Aguado et al. 2007). Chemical recycling of PET hasbeen more successful, as de-polymerization undermilder conditions is possible. PET resin can bebroken down by glycolysis, methanolysis or hydrolysis,for example to make unsaturated polyester resins(Sinha et al. 2008). It can also be converted backTable 2. Terminology used in different types of plasticsrecycling and recovery.ASTM D5033definitionsequivalent ISO 15270(draft) definitionsother equivalenttermsprimaryrecyclingmechanical recycling closed-looprecyclingsecondaryrecyclingmechanical recycling downgradingtertiaryrecyclingchemical recycling feedstockrecyclingquaternaryrecyclingenergy recovery valorization into PET, either after de-polymerization, or by simplyre-feeding the PET flake into the polymerization reac-tor, this can also remove volatile contaminants as thereaction occurs under high temperature and vacuum(Uhde Inventa-Fischer 2007).(f ) Alternative materialsBiodegradable plastics have the potential to solve anumber of waste-management issues, especially fordisposable packaging that cannot be easily separatedfrom organic waste in catering or from agriculturalapplications. It is possible to include biodegradableplastics in aerobic composting, or by anaerobic diges-tion with methane capture for energy use. However,biodegradable plastics also have the potential to com-plicate waste management when introduced withoutappropriate technical attributes, handling systemsand consumer education. In addition, it is clear thatthere could be significant issues in sourcing sufficientbiomass to replace a large proportion of thecurrent consumption of polymers, as only 5 per centof current European chemical production usesbiomass as feedstock (Soetaert & Vandamme 2006).This is a large topic that cannot be covered in thispaper, except to note that it is desirable that compo-stable and degradable plastics are appropriatelylabelled and used in ways that complement, ratherthan compromise waste-management schemes (seeSong et al. 2009).3. SYSTEMS FOR PLASTIC RECYCLINGPlastic materials can be recycled in a variety of waysand the ease of recycling varies among polymer type,package design and product type. For example, rigidcontainers consisting of a single polymer are simplerand more economic to recycle than multi-layer andmulti-component packages.Thermoplastics, including PET, PE and PP allhave high potential to be mechanically recycled.Thermosetting polymers such as unsaturated polyesteror epoxy resin cannot be mechanically recycled, exceptto be potentially re-used as filler materials once theyhave been size-reduced or pulverized to fine particlesor powders (Rebeiz & Craft 1995). This is becausethermoset plastics are permanently cross-linked inmanufacture, and therefore cannot be re-melted andre-formed. Recycling of cross-linked rubber from cartyres back to rubber crumb for re-manufacture intoother products does occur and this is expected togrow owing to the EU Directive on Landfill of Waste(1999/31/EC), which bans the landfill of tyres andtyre waste.A major challenge for producing recycled resinsfrom plastic wastes is that most different plastic typesare not compatible with each other because of inherentimmiscibility at the molecular level, and differencesin processing requirements at a macro-scale. Forexample, a small amount of PVC contaminant presentin a PET recycle stream will degrade the recycled PETresin owing to evolution of hydrochloric acid gas fromthe PVC at a higher temperature required to melt andreprocess PET. Conversely, PET in a PVC recyclestream will form solid lumps of undispersed crystallinePET, which significantly reduces the value of therecycled material.Hence, it is often not technically feasible to addrecovered plastic to virgin polymer without decreasingat least some quality attributes of the virgin plasticsuch as colour, clarity or mechanical properties suchas impact strength. Most uses of recycled resin eitherblend the recycled resin with virgin resin—oftendone with polyolefin films for non-critical applicationssuch as refuse bags, and non-pressure-rated irrigationor drainage pipes, or for use in multi-layer appli-cations, where the recycled resin is sandwichedbetween surface layers of virgin resin.The ability to substitute recycled plastic for virginpolymer generally depends on the purity of the re-covered plastic feed and the property requirements ofthe plastic product to be made. This has led to currentrecycling schemes for post-consumer waste that con-centrate on the most easily separated packages, suchas PET soft-drink and water bottles and 塑料回收英文文献和中文翻译(4):http://www.youerw.com/fanyi/lunwen_31406.html